An example of an implantable cardioverting device is in Pat. No. 3,952,750 to Mirowski et al
This device uses an AGC (Automatic Gain Control) for sensitivity adjustment. An analog signal is used as a basis for adjustment Signal averaging is performed on peak amplitudes and the sensitivity threshold is automatically adjusted by this process. However, this device does not perform any bradycardia support and was designed to detect VF (ventricular fibrillation) only. That is, it was not designed for detection of both VF and sinus rhythm and does not operate effectively as a detection device for bradycardia support and normal sinus rhythm
A problem also exists in AGC devices where a VF has a variable amplitude The presence of occasional peaks in its waveform in addition to lower "sub threshold" amplitudes is known to automatically adjust on the basis of the peaks with the effect of ignoring the lower amplitudes of the waveform, hence producing an erroneous result. Thus, an incorrect sensitivity adjustment is performed. VF is therefore undetected and could be mistaken for sinus rhythm This may cause severe difficulties, discomfort and may even cause the death of the patient.
If all VF signals were low peak, the AGC would give a correct result, but it does not allow for the existence of occasional high peaks. Thus the device could erroneously operate on the basis that sinus rhythm had been restored. A further problem with AGC devices is that they fail to detect rapidly changing amplitudes which are often observed during VF.
Furthermore, AGC devices are not generally designed to cope with bradycardia support or the condition of asystole.
If there is no signal response and asystole is present, an AGC device could automatically increase detector sensitivity until unwanted noise signals are picked up, such as muscle noise, electrical noise, etc., and these could be recognized by the device as being an arrhythmia The device would then cause the application of incorrect therapy, causing great discomfort and possibly death of a patient. This is a reason why the device has not generally been implanted in patients suffering from the effects of asystole and who need a device designed for bradycardia support.
As a result of these shortcomings, an AGC has not been effective in detection in implantable devices in patients who suffer from asystole and require bradycardia support, in addition to antitachyarrhythmia therapy In addition to antitachyarrhythmia devices, there are some existing pacemaker devices which include an AGC for sensitivity adjustments. These pacemaker devices may be effective in organized heart rhythm detection, but are not designed to allow for VF detection or for differentiation between VF and asystole.
Another prior art detection device is described in U.S. Pat. No. 4,184,493 to Langer et al and relates to VF detection using the principle of a probability-density function. In this device, the ECG is filtered by a high pass filter after which the filtered ECG is used to derive the control voltage for an automatic gain control circuit. However, this device does not overcome the problem of detection of lower amplitude or fine VF's due to the high pass filtering of the VF signal Furthermore, the device is not designed for bradycardia support of patients experiencing asystole
An article in "The American Journal of Cardiology" Vol. 52, page 265, entitled "The Automatic Implantable Defibrillator: Local Ventricular Bipolar Sensing to Detect Ventricular Tachycardia and Fibrillation" by Winkle et al. refers at page 270 to the use of rate detection circuits in conjunction with a morphology dependent criterion, such as the probability-density function to minimize the possibility of delivery of shocks during sinus or other narrow-QRS SVT. The article then states that the "addition of a morphology-dependent criterion is done at the expense of increasing the likelihood that some VT's will not be recognized by the system" and that the device "may occasionally deliver a shock for a a sinus or other SVT". This further emphasizes the need for a device which reliably detects VT and sinus rhythm and is also designed for bradycardia support pacing
Another prior art heart rate detection device is shown in U.S. Pat. No. 4,393,877 to Imran. This heart rate detection apparatus includes two mutually inclusive detector circuits responsive to ECG waveforms of different characteristics. One detection circuit is responsive to ECG waves with high slew rates, or spiky waves, while the other detection circuit is responsive to ECG waves with low slew rates, or sinusoidal waves. This device, however, is not designed to distinguish between low amplitude VF and asystole. Also the device is not designed to overcome the problem of double sensing after a defibrillation shock due to far-field R-waves or current of injury T-waves which may occur, for example when, post shock, the T-wave is, for a period of time, of a higher amplitude than the R-wave.
Furthermore, some attempts have been made at permanently increasing the sensitivity to overcome the problem of loss of low amplitude VF signals. This has resulted in the additional problem of double-sensing of VT and sinus rhythm signals, causing erroneous results in the detection device which can produce an incorrect therapy, causing great discomfort and possibly patient death.
Another problem with existing detection devices can be caused by the presence of a current of injury T-wave. It is known that after therapy (shock or pacing) when a tachyarrhythmia has been reverted and sinus rhythm has been restored, the T-wave has, for a time, an amplitude which may be as high as or higher than that of the R-wave, thereby causing a double counting effect Hence there may be at times a need in antitachyarrhythmia devices to differentiate between R-waves and current of injury T-waves in initial post therapy sinus rhythm.